1. Field of the Invention
The invention relates generally to switching device and more specifically relates to methods and structure to reduce the congestion and complexity of integrated circuit (IC) die layout for a switching device such as a Serial Attached SCSI (SAS) expander.
2. Discussion of Related Art
A number of communication media and protocols utilize switching devices to controllably route one end device to another end device to establish a temporary connection between the end devices through the switching device. In the context of a SAS domain, the Service Delivery Subsystem (e.g., the “switched fabric”) comprises one or more SAS expanders to couple a SAS initiator device to an identified SAS target device for purposes of a desired connection to exchange information.
Switching devices generally comprise a plurality of physical links (e.g., SAS PHYs) that serve as the communications interface between a first device coupled with a first physical link and a second device coupled with a second physical link. Such switching devices (e.g., SAS expanders) further typically comprise a complex switching circuit able to connect any physical link to any other physical link of the switching device. Often, a so-called “crossbar switch” is used for such controlled connections. Often the switching device (including the switching circuit) is designed as a single IC die. Such switching circuits typically comprise a large number of connection paths (i.e., conductive traces routed within the IC die) to allow such coupling of any link to any other link of the switching device. A switching device may comprise any number (N) of such physical links and thus the number or conductive traces will typically be at least N. Routing a large number of such conductive traces in an IC die can present a number of problems. The routes (conductive traces passing through the switching circuit) must be approximately equal in length so that there are not significantly different propagation delays in different connections between physical links. Further, although typical computer aided engineering (CAE) tools assist in this routing function, the complexity of resolving this routing problem increases as the number of such traces increases. The problem is often referred to as “congestion” in that there are numerous timing and IC die area constraints in the design and layout of the IC for such switching circuits.
A variety or presently known designs may all be referred to herein as “crossbar switches”. Common crossbar switch designs include: multiple Banyan switches in a “Butterfly” combination or large multiplexer configurations. All such present crossbar switch designs incorporate a large number of internal traces for routing within the switching circuit. In general, where the number of physical links is “N”, present crossbar switch designs include N2 communication paths (i.e., internal conductive traces of the crossbar switch) to permit coupling of any of N physical links to any other of the N physical links. Such a significant number of internal communication paths causes complexity and congestion in the layout of the IC die for the switching device.
Thus it is an ongoing challenge to reduce the complexity and hence congestion in the routing of traces for switching circuits within a switching device such as a SAS expander.